While Catalyst is a very powerful and flexible web app framework, many pieces end up being recoded for each new application: user interfaces, in particular.

Meanwhile, advances in Perl metaprogramming (especially Moose) have opened up new possibilities in terms of introspection. Why not build user interfaces based on class metadata? This would allow the user interface templates to be very general where possible.

Many programmers are now comfortable using ORMs of one sort or another - DBIx::Class, Class::DBI, Hibernate, etc. These systems are wonderful for bridging from the world of OO into the world of relational databases (or other datastores). This model is sometimes called a "domain model", because it models the "nouns" of a problem domain in the real world. Domain models are easily shared accross applications, and can enforce validation and other integrity constraints.

However, over time, many application developers find themselves adding business logic to the domain model. This business logic is often application-specific, and reduces the reusability of the domain model. Worse, business logic becomes spread between the model and the contoller.

Reaction adds another layer, the Interface Model. Interface models provide an adaptor to the domain model, customized for a particular application (or group of use cases). This decouples the domain model from the application, allowing it to be reused more freely. Additionally, the Interface Model becomes the natural location for business logic.

Happily, Reaction again uses reflection to make the degenerate case easy - when your IM has no customized functionality, it can simply delegate all work to the DM. When you need to add custom business logic, you can add or replace functionality as needed.